This invention relates to cast strip made by a twin roll caster, and the method and apparatus for making such cast strip.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls.
The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures on the order of 1400° C. or higher. If exposed to normal atmosphere, it would suffer very rapid scaling due to oxidation at such high temperatures. Therefore, a sealed enclosure is provided beneath the casting rolls to receive the hot strip and through which the strip passes on the way from the strip caster, the enclosure containing an atmosphere which inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust gases which may be reducing gases. Alternatively, the enclosure may be sealed against ingress of oxygen containing atmosphere during operation of the strip caster. The oxygen content of the atmosphere within the enclosure is then reduced during an initial phase of casting by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
Cast strip produced by twin roll caster is generally hot rolled in a hot rolling mill after the strip emerges from the caster to shape the thin strip. It is generally understood that to cast strip with acceptable shape, the hot rolling mill is used in connection with a twin roll caster is to provide the desired cross-sectional profile to the cast strip. Still, a surface roughness of 6 to 8 microns Ra with surface micro-cracking was common for cast strip emerging from the hot rolling mill while casting at a standard casting speed of 80 m/min and 16% reduction of the strip by the hot rolling mill. FIG. 1 is a micrograph showing typical surface roughness of such cast strip emerging from a hot rolling mill in-line with a twin roll caster. With the direction of rolling from left to right, the micrograph shows pronounced lapping on the strip surface (20 to 30 μm deep). The reason or reasons for this surface roughness may be shearing at the strip surface caused by welding of the strip to the work roll surface, imprinting of the texture of the work roll surface onto the surface of the strip, and/or other factors. Moreover, micro-cracking on the surface of the cast strip was a problem. It was possible to reduce the casting speed and the heat rate of the strip to reduce microcracking, but it was uneconomical to reproduce these conditions during production.
The microstructure of hot strip mill products is essentially 100% equiaxed ferrite. However, in making a cast strip with a twin roll caster, previous experience was that microstructure was coarse grains of polygonal ferrite, acicular ferrite, and Widmanstatten. It was typical that the microstructure was 30-60% polygonal ferrite, 70-40% Widmanstatten and acicular ferrite. With this microstructure, the typical surface roughness was 4-7 microns Ra.
A thin cast strip is provided having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra and a scale thickness of less than about 10 microns made by the steps comprising:
a. assembling a twin roll caster having laterally positioned caster rolls forming a nip between them and a hot rolling mill having work rolls and back-up rolls adjacent the twin roll caster,
b. forming a thin cast strip from the nip between the casting rolls of the twin roll caster,
c. applying a mixture of water and oil on the back-up rolls of the hot rolling mill,
d. passing the thin cast strip at a temperature of less than 1100° C. through the hot rolling mill while the mixture of oil and water is applied to the work rolls, and
e. shrouding the thin cast strip from the casting rolls through the hot rolling mill in an atmosphere of less than 5% oxygen forming a cast strip having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra, and a scale thickness of less than about 10 microns.
A thin cast strip may have a surface roughness less than 1.0 microns Ra or less than 0.7 or 0.5 microns Ra. The thin cast strip may have a scale thickness less than 7 or 4 microns. The cast strip may be passed through the hot rolling mill at a temperature less than 1050° C. while a mixture of oil and water is applied to the back-up rolls. The mixture of oil and water may be applied by spraying the upstream and/or low stream surfaces of the upper and/or lower back-up rolls. The mixture of oil and water may be less than 5% oil to form a thin cast strip with a low surface roughness of less than 1.5 microns Ra.